1. Field of the Invention
This invention relates to fabrication techniques for thermoelectric power generator modules.
2. Description of the Prior Art
The need to convert solar radiation into more practical forms of energy, such as electrical power and heat, has become apparent recently. This need has resulted in a dramatic increase in the amount of research directed to more efficient and economical methods of converting solar energy into these other energy forms.
One major area of investigation is that involving the use of photovoltaic cells which generate electrical energy directly from sunlight. Typically, photovoltaic cells are based on a semiconductor layer having an ohmic contact on one side, a rectifying contact on the other side. Despite great advances, photovoltaic cells still require near crystal perfection for high efficiency operation which means that elaborate procedures must be used in producing these devices. Because of this, photovoltaic cells have not found wide use in terrestrial applications where cost of production is a major factor.
An alternative technique for generating electrical power from solar energy involves the thermoelectric or Seebeck effect. In this technique, a thermocouple is formed by connecting a pair of n- and p-type semiconductor materials with an electrical conductor at two junctions which are maintained at different temperatures. This produces an EMF between the two junctions which is a function of the temperature difference and the thermoelectric characteristics of the materials forming the thermocouple. The EMF can be increased by connecting more than one thermocouple in series, in which case the EMF becomes proportional to the number of thermocouples connected in series.
Solar energy can be used to heat one junction of a thermoelectric device to a temperature higher than the other. This provides a convenient way to convert solar radiation into electrical energy. Some attempts to fabricate thermoelectric power generators suitable for use in converting solar energy into electrical power are described in issued patents, including: Shaffer, U.S. Pat. No. 2,984,696; Stearns, U.S. Pat. No. 3,053,923; and Liphis, U.S. Pat. No. 3,088,989. Despite proposals such as those set forth in these patents, thermoelectric converters have not been widely accepted for use in solar energy conversion because they have suffered from a number of problems. Some of the most serious problems relate to the difficulty of fabricating arrays of thermoelectric elements because the materials required, such as bismuth telluride and lead telluride, have oriented or partially oriented crystals which inevitably make them brittle and difficult to prepare. For example, great care must be taken during any cutting operation required in fabricating thermoelectric power generators from such materials because they are so susceptible to damage. Furthermore, the difficulty of forming suitable junctions increases with the number of junctions to be formed, and many junctions are required if practical amounts of power are to be generated from solar energy.